Such hammers will normally have a housing and a hollow cylindrical spindle mounted in the housing. The spindle allows insertion of the shank of a tool or bit, for example a drill bit or a chisel bit, into the front end thereof so that it is retained in the front end of the spindle with a degree of axial movement. The spindle may be a single part or may be made of two or more parts, which together form the hammer spindle. For example, a front part of the spindle may be formed as a separate tool holder body for retaining the tool or bit. Such hammers are provided with an impact mechanism which converts the rotational drive from an electric motor to a reciprocating drive causing a piston, which may be a solid piston or a hollow piston, to reciprocate within the spindle. The piston reciprocatingly drives a ram by means of a closed air cushion located between the piston and the ram. The impacts from the ram are transmitted to the tool or bit of the hammer, optionally via a beatpiece.
Some hammers can also be employed in combination impact and drilling mode or in a drilling only mode in which the spindle, or a forwardmost part of the spindle, and hence the bit inserted therein will be caused to rotate. In the combination impact and drilling mode the bit will be caused to rotate at the same time as the bit receives repeated impacts. A rotary drive mechanism transmits rotary drive from the electric motor to the spindle to cause the spindle, or a forwardmost part thereof to rotate.
In smaller hammers, a wobble drive arrangement is generally used to convert a rotary drive from the motor to the reciprocating drive of the piston. In a known arrangement the rotary drive from the motor is transmitted to an intermediate shaft mounted within the hammer housing generally parallel to the axis of the spindle. A wobble sleeve is rotatably mounted on the intermediate shaft. The wobble sleeve is formed with a wobble race which extends around the wobble sleeve at an oblique angle to the axis of the intermediate shaft. Balls are set to run between this inner race and an outer race of a wobble ring, which wobble ring has a wobble pin extending from it to the rearward end of the piston.
The wobble pin reciprocatingly drives the piston via a trunnion arrangement, which generally includes a pair of spaced arms extending rearwardly of the main body of the piston and a cross bolt. Each arm is formed with a through hole and the cross bolt is received through said through holes. A central portion of the cross bolt, lying between the two arms is formed with a through hole, through which the end of the wobble pin remote from the wobble ring extends. During hammering the wobble pin is reciprocatingly driven forwardly and rearwardly, with respect to the longitudinal axis of the spindle by the wobble drive arrangement. The wobble pin reciprocatingly drives the piston via the cross bolt and the piston arms. As the wobble pin reciprocates its orientation changes which causes the cross bolt to rotate, about the longitudinal axis of the cross bolt, with respect to the through holes of the piston arms. Also, to a limited extent the cross bolt is caused to reciprocate laterally along the direction of the longitudinal axis of the cross bolt with respect to the through holes of the piston arms. Accordingly, there is a significant amount of wear between the cross bolt and the piston arms.
One way of reducing this wear is by locating a U-shaped washer arrangement made from a bent strip of spring steel in the space between the piston arms. The U-shaped washer has a base from which the two arms extend to form the U-shape so that plane faces of the strip abut the base and the inwardly directed faces of the piston arms. Each arm of the washer is formed with a through hole, so that when the washer is fitted in the space between the piston arms, with the base of the washer abutting the rearward surface of the piston main body, the through holes in the washer arms align with the through holes in the piston arms. Then the cross bolt is fitted through the through holes in the washer arrangement as well as the through holes in the piston arms. The U-shaped washer has the advantage that it is easy to assemble onto the piston, however, it has the disadvantage that it is not free to rotate with the cross bolt, which is not ideal for the reduction of wear.